The present invention relates to a wire bonding method for connecting a chip electrode on a semiconductor chip and a lead of a lead frame through a metal wire.
A conventional wire bonding method for connecting the chip electrode and the metal wire comprises the steps of pressing the metal wire against the chip electrode and transmitting an ultrasonic oscillation to the metal wire after contact between the metal wire and the chip electrode.
Generally speaking, the surface characteristics of the chip electrode vary greatly according to the kind of semiconductor chip.
For chip electrodes having a hard surface layer, a larger ultrasonic oscillation is required to securely perform wire bonding. However, in this case, a shortcircuit problem and imperfect bonding problems occur because the contact area of the metal wire is increased by the impact of the metal wire upon the hard chip electrode.
In order to solve the problems, a bonding wire method is adopted, where the ultrasonic oscillation is transmitted before the contact point between the metal wire and the chip electrode.
However, in this case, the larger ultrasonic oscillation suitable for the crushed metal wire is transmitted to even the metal wire having a small contact surface. Accordingly, the larger ultrasonic oscillation is transmitted to the small contact surface of the metal wire, so that cracks develop within the chip electrode.